The present invention relates to a method for controlling high-pressure of a refrigerant in an intermittently supercritically operating refrigeration circuit which circulates in use a refrigerant in a predetermined flow direction and which comprises in flow direction a compressor, a heat-rejecting heat exchanger, a control valve associated to the outlet of the heat-rejecting heat exchanger and a control for controlling the control valve.
Various methods for controlling the high-pressure in the refrigerant subsequent to the heat-rejecting heat exchanger, i.e. the gas cooler and/or the condenser, in a subcritical as well as a super critical operational condition are known. A float regulation valve is for example a well-known device for high-pressure regulation in a subcritically operating refrigeration circuit. Depending on the liquid level in the float-regulating valve, the float opens or closes an outlet opening for tapping liquid refrigerant into the refrigeration circuit, and typically to a receiver in which the refrigerant is collected and stored before its delivery to the refrigeration consumer(s). Gaseous refrigerant on the other hand will not lift the float so that the outlet opening will remain closed. Accordingly, the liquifying pressure in the heat-rejecting heat exchanger will be raised ensuring delivery of fluid refrigerant to the float-regulating valve.
In the supercritical operational mode no liquid refrigerant will be present subsequent to the heat-rejecting heat exchanger and the float-regulating valve cannot effect a high-pressure regulation. In the supercritical mode exists, however, a correlation between gas cooler outlet temperature and high-pressure of the refrigerant for which the COP (coefficient of performance) is at a maximum. The respective formula for calculating such optimum high-pressure in the supercritical mode is known to the person skilled in the art, and by means of a control valve associated to the outlet of the heat-rejecting heat exchanger and gas cooler, respectively, the refrigerant pressure can be controlled based on this formula.
Intermittently supercritically operating refrigeration circuits, for example with CO2 as refrigerant, operate intermittently in a supercritical operational mode, with the refrigerant exiting the gas cooler being in the gaseous state. They also intermittently operate in a subcritical or “normal” mode where the refrigerant exiting the condenser is in the fluid state. Whether the refrigeration circuit operates in the supercritical or in the subcritical mode depends on the temperature of the medium against which the refrigerant is cooled and condensed, respectively, in the heat-rejecting heat exchanger. As such medium is typically ambient air, the subcritical operational mode is also referred to as the “winter mode”, while the supercritical operational mode is also referred to as the “summer mode”. Alternative media may be water or brine.
The control of the high-pressure of the refrigerant in the border region next to the critical point where the supercritical operational mode transfers to the subcritical operational mode and vice versa is a problem with such refrigeration circuits.